1. Technical Field
The technical field relates to an information recording and reproducing apparatus that reads and writes data from and to a recording medium such as a nonvolatile memory.
2. Related Art
Conventionally, information recording and reproducing apparatus such as SD (Secure Digital) card as card type recording media containing a flash memory is ultracompact and ultraslim. Due to its easy handling, it is widely used for recording data in an image in digital camera, mobile device or the like.
Flash memory included in the information recording and reproducing apparatus has a lot of physical blocks each of which has a constant size, and data can be erased on a physical block basis.
In these latter days, in order to cope with the demands for enlargement of recording capacity of a flash memory, a flash memory that can store data of 2 bits or more in one cell (hereinafter, “multi-valued flash memories”) is commoditized.
FIG. 10 is a diagram illustrating one example of a relationship between the number of electrons accumulated in a floating gate of a quaternary flash memory capable of storing 2-bit (quaternary) data in one cell and a threshold voltage (Vth). As shown in FIG. 9, the quaternary flash memory manages four accumulation states of electrons by four states in the floating gate according to the threshold voltage (Vth). In a erase state of the flash memory, an electric potential is the lowest, and this state is represented by (1,1). The voltage threshold discretely rises as the electrons are further accumulated, and states of the rising steps are represented by (1,0), (0,0) and (0,1), respectively. In the quaternary flash memory, 2-bit data can be recorded in one memory cell by utilizing the rise of the electric potential in proportional to the number of accumulated electrons.
However, in the quaternary flash memory, since the four states are discriminated by an accumulation amount of the electrons, a difference in the threshold voltage among the states is smaller than that in a binary flash memory.
When rewriting of data to the flash memory is repeated, a gate oxide film is slightly deteriorated by injection drawing of the electrons to and from the floating gate. This deterioration is repeated, so that a lot of electron traps are formed and thus the actual number of electrons to be accumulated in the floating gate is decreased. Particularly when a semiconductor process is miniaturized, the number of the electrons to be accumulated in the floating gate decreases, and thus an influence of the electron traps becomes great.
A problem of the deterioration in data retaining characteristics in the flash memory becomes noticeable according to the multi-valued recording and the miniaturization of the semiconductor process that support the enlargement of the capacity of the flash memory.
A method for solving the above problem includes:
(1) restriction of the number of times of rewriting; and
(2) strengthening of error correction.
JP-A-2006-18373 discloses a flash memory that strengthens an error correcting ability and enables the restriction of the deterioration in the data retaining characteristics. In JP-A-2006-18373, in the flash memory including a plurality of chips, blocks in respective chips of the flash memory are related. The plurality of related blocks is treated as one group of the blocks, and one block in the group are allocated as a block where parity data related to user data written to another block in this group is recorded.
In the method described in JP-A-2006-18373, RAID 5 used in a hard disc drive (HDD) is applied to a flash memory. However, when this method is applied to a semiconductor memory such as a flash memory, redundancy is increased by parity data, thereby deteriorating a transmission performance of the data. Further, a power consumption is increased by a writing amount of the parity data. That is to say, the high-speed transmission and the lower power consumption cannot be fulfilled.
Particularly in applications for business purpose that record moving images in memory cards, the transmission performance may satisfy real-time recording at the time of writing, but a high-speed transmission performance so as to enable high-speed uploading of the recorded data to servers, for example, is required at the time of reading.